diff --git a/src/storage.cr b/src/storage.cr
index c1356fa..8c56419 100644
--- a/src/storage.cr
+++ b/src/storage.cr
@@ -112,6 +112,9 @@ class DNSManager::Storage
 
 		user_data = user_must_exist! user_id
 
+		# Prevent future very confusing errors.
+		domain = domain.downcase
+
 		return Response::DomainAlreadyExists.new if zones_by_domain.get? domain
 
 		# Verify if the domain is acceptable.